This invention relates to hot melt pressure sensitive adhesive compositions, particularly to hot melt pressure sensitive adhesive compositions suitable for coating on tapes or labels and most particularly on tapes to be used as closures for disposable diapers.
Disposable diapers typically comprise an absorbent filler material and an outer moisture-impervious polyethylene film. The diapers are generally so configured that when they are placed on an infant, they are folded so that two adjacent edges are either juxtaposed or overlapped, with a strip of normally tacky and pressure-sensitive adhesive tape being used to hold the edges together.
Hot melt adhesives utilized heretofore for such applications have been based primarily on block copolymers, particularly those of the styrene-isoprene-styrene types, which copolymers are blended with tackifying resins, oils and antioxidants to provide the final adhesive product. Typical of such adhesives are those described in U.S. Pat. No. 4,163,077.
While the adhesives based on the styrene-isoprene-styrene copolymers possess good adhesion to a wide variety of substrates including the polyethylene diaper films, the adhesives are sensitive to oxidation and degradation both during processing and in use. Other adhesive compositions, based on partially hydrogenated block copolymers are known to possess improved resistance to oxidation and degradation but are deficient when formulated into adhesives which are to be used in adhering polyolefin substrates.
It is an object of the present invention to provide hot melt pressure sensitive adhesive compositions which exhibit a combination of advantageous properties including adhesion to polyolefin substrates together with superior heat and oxidative resistance.